One of the more pernicious problems is security checks written to detect underhanded violations of constraints being eliminated because the compiler doesn't recognize the underhandedness is possible.

In a past life, I depended on signed overflow to detect out of bounds memory allocations. That kind of stuff gets compiled away these days, you need to be cleverer to detect when the user is deliberately trying to invoke UB. And because the compiler doesn't believe in UB, there's the risk it's going to remove your detection logic.